Haile Gebrselassie poses next to his new world record time in the marathon. Photograph: Getty Images/Bongarts

One of the mighty unscaled peaks of athletics lost a little more of its once presumed invincibility last Sunday when Haile Gebrselassie gave the world a glimpse of the first sub two-hour marathon. More than four decades since the sport's last great conquests - Roger Bannister's destruction of the four-minute-mile barrier in 1954 and Jim Hines' breaking 10 seconds for the 100 metres in 1968 - the same lasting legacy achieved by these two men now beckons a long-distance runner.

A week ago, Gebrselassie, the Ethiopian runner who would be precious little use in a pub brawl but with a bird-like frame, long stride and voracious oxygen uptake that make him a super-efficient distance runner, surpassed even the high expectations that preceded his run over the fast, flat course in Berlin. His time of two hours three minutes 59 seconds sliced 27 seconds off his own world marathon record and, by breaking through the 2:04 barrier, scattered the clouds that had obscured the sub-two-hour peak.

Expert opinion is deeply divided over how long it will take before the summit is reached, and some even remain doubtful whether it ever will be. Plenty of authoritative voices say otherwise, though, none with greater conviction than David Bedford, the former world-record holder for 10,000 metres and now race director of the London Marathon. 'Without doubt I will see a two-hour marathon in my lifetime,' Bedford, who is 58, says. 'It might be towards the end of my life. It might be another 20 years. But, yes, it will definitely happen.'

Bud Baldaro, who has been UK Athletics' marathon coach, agrees with Bedford. 'I think it will happen now,' he says. 'They have been getting significantly closer for the past four or five years and I think that in the next 20 years someone who has the running economy and track-running pedigree of Gebrselassie or Kenenisa Bekele [the world 10,000m record holder] will do it.'

The doubters include Ron Hill, who in 1970 was probably the first runner to go under the 2hr 10min mark, the only other sub 2:10 time having been over a course now reckoned to have been short of the full distance. He maintains his position despite having once said 2:05 could not be beaten. 'I said that would be the limit before the altitude runners began to show their potential,' Hill says. He also cites pre-race diet, in-race fuel supplies and pacemakers, unheard of in his time, as reasons for the record now being faster than he previously thought possible.

Hill might have thrown in the considerable incentive provided by financial rewards. Gebrselassie earned £102,000 plus an undisclosed appearance fee for his third win in Berlin.

Glenn Latimer, one of the world's leading marathon authorities who oversees long-distance running in the United States, is as sceptical as Hill about the likelihood of anyone breaking through the two-hour barrier, describing it as 'a far-off dream'. He was in a lead vehicle watching Gebrselassie run in Berlin last weekend, and says: 'You could tell from Haile's face over the last four or five kilometres that it was hard work. Although his form remained brilliant and he was very smooth, he was showing stress.

'If you look at what his splits were, averaging around 14 minutes 45 seconds for each five kilometres, they're amazing. You're talking something else altogether to go down significantly below this.'

Latimer says he would be staggered if a time of under two hours were ever recorded. Ever? 'They'd have to invent some very good drugs for it to happen because we know what happens to the body after 30 kilometres. It really starts to suffer and break down.'

Gebrselassie, 35, himself a believer in the two-hour marathon - 'Maybe in 20 years, maybe 40 years. The more technology develops, the more athletes will run faster' - reeled off kilometres at between 2min 53.8sec and 2:58.2 as he broke the record he set a year ago in the same race. There have been seven world records in the event - four men's, three women's - whose date in late September more or less guarantees a perfect temperature (this year between 10C at the start and 14C at the finish). 'The course is fast, the weather was perfect and the rain the day before made everything fresh,' Gebrselassie said. 'And the pacemakers were good. You don't often get all these things together.'

The luxury of pacemakers referred to enviously by Hill and the unexpectedly strong run by Kenya's James Kwambai, who lowered his personal best by nearly five minutes in finishing second in 2:05:36, were important factors in Gebrselassie's run.

Although only one of four pacemakers who were there to ensure a world-record tempo, the Kenyan Abel Kirui, survived the relentlessly high pace beyond 30km, before being dropped at 32km, Kwambai clung on and appeared briefly to be capable of staying with Gebrselassie until the finish. 'Haile looked over and you could see him wondering, "Who is this guy?",' Latimer says. Eventually, though, even Kwambai was burned off with just over 5km to go, which had the positive effect of relieving Gebrselassie of having to engage in a tactical battle over the closing stages.

With just the clock to beat as the finishing line approached, Gebrselassie wound up the pace to produce his fastest 5km split between the 35km and 40km marker - a savagely quick 14min 29sec - which carried him to his 26th world best, over distances ranging from two miles to the marathon, in 14 years.

Put in terms of seconds per kilometre, the task of raising the pace from the one Gebrselassie ran last Sunday to the one that would be needed to complete a two-hour marathon does not seem especially daunting: 2min 55sec pace would have to come down to 2:51. Expressed in terms of distance, though, and a mere four seconds seems very different. It is around 24 metres per kilometre, which would be regarded as a substantial winning margin in a 1,000m race.

It is a figure that places Alan Storey, the UK Athletics' senior performance manager, on the side of those who believe two hours for the marathon is, as he puts it, 'somewhere between very unlikely and impossible'.

Storey alludes to the old brain-teaser about how many jumps it would take to reach a tree if every time you leapt forward you covered half the distance of your previous jump. The answer is you would never reach the tree. For tree read two-hour marathon and the same answer applies, Storey reasons, to the question of how many attempts it would take to gain those 24 metres per kilometre.

'Given all of the science that we have available now, I have seen nothing to suggest another huge improvement could be made,' Storey says. 'If some exercise physiologist discovers something new and exciting then anything could be possible, but given all of the information we know about I don't expect to see two hours broken in my lifetime - and I'm a youthful 63.'

Asked about what new and exciting things might be out there, Storey says: 'If I had any idea what they were I'd have people working on them now - under wraps so that we could use them in 2012. No doubt these physiologists are trying to find ways to cope with the stresses of running marathons, but I don't know anybody from a science background who thinks two hours is likely.'

David Bedford might not claim to be from a scientific background, but he was an extraordinary runner who lowered the world 10,000m record by 7.6sec in 1973 when he ran 27min 30.80sec, a time that only three British runners have beaten in the 35 years since. He became so addicted to training that he covered more than 200 miles per week. 'It made me incredibly strong,' he says, 'but it also kept me injured a lot of the time.' This is the experience on which he draws to make the argument, contrary to Storey's and others, that it is not whether but when the 120 minutes for the marathon will be breached.

Bedford believes it will be achieved simply through what he calls a continuing evolution of times. 'For example,' he says, 'if you take Kenenisa Bekele as the No1 10,000-metre runner at the moment, he is significantly faster than Haile Gebrselassie over the distance - so therefore I believe that when he and his generation move up to the marathon we will start to see times like two hours two-and-a-half minutes or even two hours two minutes. And this will continue. So what I think we are talking about is maybe three generations from now athletes getting it into their heads that it is possible.

'You need to look back to the previous most famous barrier, the four-minute mile. For a long time people didn't think it was possible and then all of a sudden one whole generation started to believe it could happen and started to work towards it. But that generation had to be near enough to it to sense that it was possible.

'Although Gebrselassie and Bekele are amazing, I don't think their generation of distance runners is close enough to two hours for the marathon to see it as possible. However, they will continue the erosion of times and a future generation, two or three down the line - so you're talking 10 to 15 years - will be the first to run two hours one-and-a-half minutes, or something like that, and will start believing that two hours can be done.'

Believing you can break a record as a necessary prerequisite to actually breaking it is something that Tim Noakes, the South African doctor and long-distance runner, has spoken about. In reference to Bannister's four-minute mile, Noakes has said: 'He was able to convince his brain that it could achieve what none had done before.'

Bedford backs up his argument by citing the one-hour barrier for the half marathon, which withstood all attempts at it until Moses Tanui of Kenya ran 59min 47sec in Milan on 3 April 1993. Since then Tanui has been joined by 34 others who between them have gone under 60 minutes on a further 69 occasions. 'That again was something that was viewed as not possible,' Bedford says. 'As soon as people start running 55 minutes for a half marathon [the record stands at 58:33], the world will be ready for the first two-hour marathon.'

The assumption that a man will be the first to run two hours is not necessarily safe. Bedford, though, forthrightly declares that a woman will not do it for at least 250 years, while Bud Baldaro believes that it will be done by 'someone with a very simple lifestyle'.

'Ethiopian and Kenyan runners have shown over the past three decades what can be achieved and I wonder what might come out of Asia and South America when the world changes even more,' Baldaro says. He doubts US or European runners will be involved. 'The Western lifestyle is so complicated and complex now that the guys find it very difficult to sacrifice everything to run 150, 160 miles a week.'

As to where the first sub two-hour marathon might be run, Berlin in the autumn is the obvious answer, although Bedford has not given up on London staging the historic event.

'Remember that Paula Radcliffe's two-fifteen-twenty-five, which stands above and beyond what anyone else has done in the women's marathon, was done in London,' he says, 'and also Khalid Kannouchi did two-five-thirty-eight in the men's event in 2002. So on the right day London is certainly able to have a world record again.'

And could it be that in 20 years' time David Bedford will still be the race director who secures the runner who makes history through the streets of London? 'I think in 20 years' time I will have spent the morning of the marathon in the hospitality tent and the afternoon sound asleep,' Bedford says.